The present invention relates to a magnetic tape cassette and, more particularly, to a magnetic tape cassette having a tape guide block which is arranged between top and bottom cover plates for guiding a tape along an open front of the tape cassette.
A conventional magnetic tape cassette is usually formed by a two-piece plastic case that is assembled by joining top and bottom half cases of substantially the same configurations, which are made of molded plastic material. However, such plastic half cases having rather complicated configurations tend to warp due to thermal deformations and molding strains of the plastic material. Accordingly, such an assembled case often has problems, such as an irregular tape run and an off-track or out-of-phase recording or playing, in the tape cassette.
In order to remove these problems, to prevent the warping of the cassette case, to improve the acoustic performance of the tape cassette and to increase the mechanical strength of the case, a three-piece cassette case has been proposed in U.S. Pat. Nos. 4,267,986 (Uemura et al.) and 4,330,068 (Shoji), assigned to the same assignee as this application.
However, in the proposed cassette cases comprising top and bottom cover plates and an intermediate frame inserted therebetween, the intermediate frame having a thin rectangular ring form is provided with a plurality of projections on its front for guiding a tape. Hence, the intermediate frame functions not only as a frame but also as a tape guide. As a result of its function as a frame, both prevention of the deformation of the case and an increase in the mechanical stregth and weight of the case may be attained. Further, it has been proven that such an increase in the weight of the case prevents unnecessary resonance or vibration, thus resulting in better acoustic performance.
As a result of its function as a tape guide, a proper friction is imparted between the running tape and the contact surface of the tape guide part, which is essential for properly determining the shape of the tape guide part and increasing the dimensional accuracy of the tape guide part.
In the prior art, the intermediate frame is required to function both as a frame as well as a tape guide, but the functions of these two members sometimes contradict each other. In other words, in this case, the combined functioning as the frame and as the tape guide can only be attained to a limited degree or by conflict with one another at the same time.
In U.S. Pat. No. 4,506,846 (Gelardi et al.), a tape cassette comprising top and bottom plastic half cases and a separate tape guide for guiding a tape along the open front of the tape cassette is disclosed. In this case, the tape guide is made in a simple form so as to attain an improvement of the dimensional accuracy of the tape guide part. However, since the cassette case is mainly composed of the top and the bottom plastic cases, there are problems in the functioning of the frame even when the tape guide is functioning very well. For instance, the insufficient mechanical strength of the case may cause it to bend. Further, the insufficient weight of the case is liable to cause unnecessary resonance or vibration, thus giving rise to acoustic performance problems.
Further, as described above, in the function as a tape guide, it is necessary to impart proper friction to the running tape from the contact surface of the tape guide part. However, in the tape guides disclosed in the aforementioned U.S. patents, it is very difficult to impart proper friction to the running tape in the tape guide part. Accordingly, in the tape insertion part of the tape cassette of the prior art, an irregular tape run occurs and thus the tape cannot travel smoothly in the tape guide part. Hence, an unclear sound phenomenon in the frequency modulation or the modulation noise often occurs.